1. Technical Field
The present invention relates to an infrared sensor and an infrared sensor array, in particular, to a thermal type infrared sensor and a thermal type infrared array sensor including a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) sensor which operates in a subthreshold region.
2. Description of the Related Art
For example, Japanese Patent Application Publication 2006-258562 describes a thermal type infrared sensor including a MOSFET which operates as a temperature detector in a subthreshold region.
FIG. 11 provides a circuit diagram describing a configuration of a conventional thermal type infrared sensor.
The infrared sensor includes a MOSFET sensor 101 and a constant current source 103. The MOSFET sensor 101 is driven by a constant current Id applied from the constant current source 103, operates in a subthreshold region, and outputs a sensor output voltage Vout.
The MOSFET 101 sensor is formed on a heat-insulated structure for improving sensitivity to infrared light incidence. In this case, the heat-insulated structure is configured to thermally separate the MOSFET sensor and a substrate. In general, the heat-insulated structure is configured to support a thin-film membrane portion in which the MOSFET sensor 101 is formed in a hollow state with a plurality of beams.
FIGS. 12A, 12B provide schematic views each describing one example of a heat-insulated structure. FIG. 12A provides a plan view and FIG. 12B provides a sectional view in the A-A′ position of FIG. 12A.
The heat-insulated structure is configured to support a membrane portion 105 with two beams 107. A space 111 is formed between a substrate 109 and the membrane portion 105.
The MOSFET sensor 101 is formed in the membrane portion 105, and is thermally separated from the substrate 109.
If infrared light enters the membrane portion 105, the temperature of the membrane portion 105 is increased, so that the temperature of the MOSFET sensor 101 formed in the membrane portion 105 is also increased. If the temperature of the MOSFET sensor 101 is changed, the threshold voltage of the MOSFET 101 is changed, and this change is obtained as a change in the sensor output voltage Vout. Namely, the thermal type infrared sensor uses the MOSFET sensor 101 as a temperature sensor, and captures a minute temperature change by the infrared light incidence so as to detect infrared light.
However, the thermal type infrared sensor has a problem in that the DC (direct-current voltage) level of the sensor output voltage is changed due to the effect of the ambient temperature. Namely, since the basic principle of the thermal type infrared sensor is a temperature sensor, if the temperature of the MOSFET sensor 101 is changed due to the change in the ambient temperature except for the infrared light incidence, the sensor output voltage Vout is changed. The MOSFET sensor 101 requires a large temperature coefficient of a threshold voltage in order to improve sensitivity to infrared light. However, the MOSFET sensor 101 also has a problem in that if the temperature coefficient of the threshold voltage is increased, the change in the sensor output voltage Vout due to the effect of the ambient temperature is increased.
FIG. 13 provides a circuit diagram describing one example of a conventional thermal type infrared sensor in which a constant current source is constituted by a P-type MOSFET 113.
With this configuration, the temperature property differs between the MOSFET sensor 101 constituted by an N-type MOSFET and the current source MOSFET 113 constituted by a P-type MOSFET, so that the DC level of the sensor output voltage Vout is changed due to a change in the ambient temperature similar to the configuration illustrated in FIG. 11.
To solve the above problem, a method of controlling an infrared sensor to a constant temperature with an electronic cooling element such as Peltier device is proposed. However, this method has a problem in that the configuration of the sensor portion becomes complex and the size of the sensor portion is increased due to a cooling element and a temperature controller being required for this method, resulting in the increase in the manufacturing costs. Moreover, this method also has a problem in that large power consumption is required for the Peliter device and the system for controlling the device, so that the power consumption of the entire system is increased.
To solve the above problem, a method of disposing a reference sensor is also proposed.
FIGS. 14A, 14B are circuit diagrams each describing an infrared sensor having a reference sensor. The infrared sensor includes a reference MOSFET sensor 115 and a constant current source 117 in addition to the MOSFET sensor 101 and the constant current source 103 illustrated in FIG. 11. The reference MOSFET sensor 115 is formed by a configuration which is the same as that of the MOSFET sensor 101, and the constant current source 117 is formed by a configuration which is the same as that of the constant current source 103. The reference MOSFET sensor 115 is formed in a position different from the heat-insulated structure. The reference MOSFET sensor 115 is driven by the constant current Id applied from the constant current source 117, operates in a subthreshold region, and outputs a sensor output voltage Vout 2. The effect due to the change in the ambient temperature can be removed based on the difference between the sensor output voltage Vout 1 of the MOSFET sensor 101 and the sensor output voltage Vout 2 of the reference MOSFET sensor 115.
By using this method, it becomes unnecessary to use a cooling device or the like. However, the DC levels of the sensor output voltages Vout 1, Vout 2 are changed due to a change in the ambient temperature. Specifically, if the infrared sensor is used in an area where the temperature sensitivity of the MOSFET sensors 105, 115 is large and the change in the ambient temperature is large, it becomes necessary to increase the power source voltage to be applied to the MOSFET sensors 101, 115 and a latter circuit which obtains the difference between the sensor output voltages Vout 1, Vout 2 in view of the DC level change in the sensor output voltages Vout 1, Vout 2 resulting from the temperature change. As described above, this method has a problem in that if the usage environment temperature area of the sensor is increased, the DC level changes of the sensor output signal and the reference temperature sensor output signal are increased, so that it becomes necessary to increase the power source voltage of the next stage circuit, and the low voltage operation is limited. This method also has a problem in that if the sensitivity of the sensor element is increased, the DC level change of the sensor output signal is increased, so that it becomes necessary to increase the power source voltage of the next stage circuit, and it becomes difficult to operate with high sensitivity and low voltage.